They also allow some insights on future demand, which can help you plan production of your commodity.
The issue is that the odds aren't actually 50/50 on you buying either side of the trade; one half will look like a better deal (and given public information, it is a better deal) so you'll buy that half. Then when the market resolves, it'll turn out that insiders knew some piece of information that made the other half of the trade a better choice.
P(W | You're a sucker) = 0.5
hard to win at a game where 97% fail in the long run.